ABSTRACT! African ancestry is a risk factor for worse outcomes in triple-negative breast cancer (TNBC). Nevertheless, prognostic tools and treatment regimens are no different for African American (AA) than European American (EA) TNBC patients. Personalizing medicine for AA TNBC patients has been hindered by the fact that this population is highly admixed, so self-reported race often does not accurately reflect African genetic ancestry. There is also a dearth of studies that have analyzed tumor biomarkers and clinical outcomes using ancestry- genotyped TNBC specimens. Furthermore, few preclinical TNBC studies consider race in experimental design, and AA TNBC patients are markedly underrepresented in clinical trials. A critical barrier to progress in improving outcomes for AA TNBC patients is a lack of prognostic tools and treatment modalities that have been precisely tailored to this patient population. The broad, long-range goal of this project is to enable development of precision medicine for AA TNBC patients by advancing knowledge of the utility of nuclear HSET (nHSET) as a prognostic biomarker for AA TNBCs as well as the molecular mechanisms of racial disparities in TNBC aggressiveness and how they can be targeted. Our lab has uncovered that HSET more strongly promotes proliferation and migration of AA than EA TNBC cell lines. In addition, nHSET independently predicts poor outcomes in AA but not EA TNBCs, but this must be validated after accounting for percent African genetic ancestry in multivariable survival models and in native African TNBCs. Our proposed project has three aims. First, the proportion of African genetic ancestry will be determined for a large cohort of TNBC patient samples acquired from US, and Nigeria and we will determine if nHSET can serve as a racial disparity biomarker for the stratification of TNBCs after adjusting for their percent African genetic ancestry. Second, since HSET and MYH9 are nuclear binding partners that may assist in chromatin modification to amplify oncogenic Wnt/?-catenin signaling and MYC expression (which we have found is enriched in AA compared with EA TNBCs), we will characterize racial differences in the HSET-MYH9-MYC axis in TNBCs using patient-derived samples and correlate these racial distinctions to differences in genomic stability in TNBCs. We will employ multi-colored lenti- CRISPR-Cas9 system for knocking out key genes in this axis in AA/EA cell lines to test if the HSET-MYH9-MYC axis drives tumor aggressiveness more strongly in AA than in EA TNBCs. In Aim 3, the value of a promising commercially available HSET inhibitor will be tested in xenograft models of AA and EA TNBC patient-derived cells. Thus, this study will test the hypothesis that nHSET is a novel therapeutically actionable biomarker with greater value for AA than EA TNBC patients that drives TNBC progression and chemoresistance more strongly in AA than EA TNBC patients. The impact of this project will be to advance knowledge of prognostic biomarkers, molecular mechanisms of disease aggressiveness, and effective treatment regimens for AA TNBC patients. !